The present invention relates to a method and an apparatus for measuring a biological condition of a living body, such as the pulse rate thereof.
Recently, needs of monitoring the heartbeat rate of a living body, such as a human body, increase during exercise, such as jogging for health maintenance.
One of usual methods of measuring the heartbeat rate is to measure an action potential along with a heartbeat of a living body from the chest region to obtain an electrocardiogram, and to calculate an interval between the adjacent amplitude peaks of the electrocardiogram wave.
This method, however, when measuring the heartbeat, it is necessary to affix at least one electrode pad on a portion of the living body, and it is tedious to carry out the affixing works of the at least one electrode pad.
It is considerable to, therefore, as one of more simplified methods, measure pulse waves in the living body to detect a pulse rate of the living body based on the measured pulse waves these days.
The pulse waves are pressure fluctuations occurring in the arteries in response to the heartbeat and propagating into the peripheral arteries as wave. As one of measuring the pulse waves, an optical pulse wave sensor is well known.
The optical pulse wave sensor uses light-absorbing characteristic of hemoglobin in blood. That is, the optical pulse wave sensor irradiates light to a measurement portion of the living body to measure the volume of fluctuating blood in the peripheral arteries. The optical pulse wave sensor is easily fittable to a measurement portion of human body, such as one of the fingers, one of the arms, or one of the temples of the human body, to measure the pulse wave thereof. The optical pulse wave sensor, therefore, will become widespread as the apparatus for detecting the pulse rate.
Concretely, division of the number of 60 by a time interval between the amplitude peaks of electrocardiogram waveform (unit: second) allows the heartbeat rate (unit: pulse per minute) to be obtained. Similarly, division of the number of 60 by a time interval between the amplitude peaks of pulse waveform allows the pulse rate (unit: pulse per minute) to be obtained.
That is, the heartbeat rate and the pulse rate are represented by the following equations:HR (pulse per minute)=60/IT1 (second)  [Equation 1]PR (pulse per minute)=60/IT2 (second)  [Equation 2]
where HR represents the heartbeat rate, IT1 represents the time interval between the amplitude peaks of electrocardiogram waveform, PR represents the pulse rate, and IT2 represents the time interval between the amplitude peaks of pulse waveform.
The timing at which each amplitude peak of the electrocardiogram waveform is represented and that at which each amplitude peak of the pulse waveform is represented are usually synchronized with each other (see “synchronous region” in FIG. 16), which shows the heartbeat rate and the pulse rate are equal to each other.
The movement of the measurement portion of the living body on which the optical pulse sensor is fitted in daily life or in exercise causes the blood flowing in the peripheral arteries to be disturbed. The disturbance of the blood flow generates other amplitude peaks of the pulse waveform that are independent of the heartbeat (see “asynchronous region” in FIG. 16) so that the heartbeat rate and the pulse rate are unequal to each other. This interferes with the use of the pulse rate in substitution of the heartbeat rate.
In addition, frequencies of other amplitude peaks of the pulse waveform that are independent of the heartbeat are close to the amplitude peaks thereof that are synchronized with the amplitude peaks of the heartbeat.
This characteristic makes ineffective to use frequency-filtering operation of the pulse wave that is applied for usual noise rejection operations.
In order to solve the above problems, Japanese Patent Publication No. H07-299044 discloses the technique using an exercise noise sensor.
In this technique, the exercise noises are sensed by the exercise noise sensor as a signal related to exercise noises, and the sensed signal corresponding to the exercise noises is removed from the signal on which the exercise noises and the pulse wave signal are superimposed, allowing a pulse wave signal to be accurately detected even during exercise.
Furthermore, Japanese Patent Publication No. H07-088092 discloses the technique using different wavelength lights.
That is, the different wavelength lights are irradiated on a measurement portion of the living body to obtain signals corresponding to the different wavelength lights. The obtained signals are signal-processed to separate the pulse-wave components in the bloods from the wave components of living body's movement therein, thereby measuring the pulsebeat components.
The technique disclosed in the former Patent Publication, however, even if the exercise noise sensor can detect the exercise noises, cannot detect noises generated related to the living body itself, such as noises generated due to the light components reflected from the surface of the measurement portion, such as the skin surface thereof.
In the technique disclosed in the later Patent Publication, however, the obtained wavelength signals include the pulse wave components in the bloods and the wave components of living body's movement therein. In addition, the relationship between the pulse wave components in the bloods and the wave components of living body's movement therein changes as dependent on the fitting condition of the sensor to the living body's measurement portion, and on the individual differences among the living bodies.
Because of the change of relationship, it is difficult for the disclosed unique signal processing in the technique to accurately obtain the pulse wave components in the bloods.